The present invention relates broadly to an apparatus for measuring the coherence of a light field, and in particular to a fiber optic coherence meter.
The design and use of optical interferometers, devices in which interference of light is used as a tool in metrology and spectroscopy. These uses include precise measurements of wavelength, the measurement of very small distances and thicknesses by using known wavelengths, the detailed study of the hyperfine structure of spectrum lines, the precise determination of refractive indices, and, in astronomy, the measurement of binary-star separations and the diameters of stars. Optical interferometers are based on both two-beam interference and multiple-beam interference. They are also based on wavefront splitting and amplitude splitting.
One of the more well known interferometers is the Michelson interferometer. The principle of operation of the Michelson interferometer is based on amplitude splitting. Light rays from a narrow-angle source are incident at 45.degree. on a 50% partially reflecting plate, P.sub.1. Half the light is transmitted through plate P.sub.1 to a first mirror, M.sub.1 which reflects the light back to the 50% reflecting plate P.sub.1. The light which is reflected by the reflecting plate P.sub.1 proceeds to a second mirror, M.sub.2 which reflects it back to the 50% reflecting plate, P.sub.1. At plate P.sub.1, the two waves are again partially reflected and partially transmitted, and a portion of each wave proceeds to the receiver, R which may be a screen, a photocell, or a human eye. Depending on the difference between the distances from the beam splitter (plate P.sub.1) to the mirrors M.sub.1 and M.sub.2, the two beams will interfere constructively or destructively. A plate P.sub.2 is sometimes positioned between plate P.sub.1 and mirror M.sub.1 to compensate for the thickness of P.sub.1.
When the mirrors' images are completely parallel, the interference fringes are circles. The reflectivity of the mirrors in the Michelson interferometer can be made as high as desired, and the interference will still be two-beam interference. The intensity of the fringes can accordingly be made very great. If the mirrors are slightly inclined about a vertical axis, vertical fringes are formed across the field of view. These fringes can be formed in white light if the path difference in part of the field of view is made zero. Just as in other interference experiments, only a few fringes will appear in white light, because the difference in path will be different for wavelengths of different colors. Accordingly, the fringes will appear colored close to zero path difference, and will disappear at larger path differences where the spectral separation of the successive regions of constructive interference is too close for the eye to see colors. If there is a one-half cycle relative phase shift at the beam splitter, the fringe of zero path difference is black, and can be easily distinguished from the neighboring fringes. This makes use of instrument relatively easy. The sensitivity to weak lines and resolution of the interferometer is thus potentially very much greater than that of an optical spectrometer.
The state of the art of interferometry is well represented and alleviated to some degree by the prior art apparatus and approaches which are disclosed in the following U.S. patents:
U.S. Pat. No. 3,639,063 issued to Krogstad et al on Feb. 1, 1972;
U.S. Pat. No. 3,879,988 issued to Jacobs on Apr. 29, 1975;
U.S. Pat. No. 4,090,793 issued to Lebduska on May 23, 1978;
U.S. Pat. No. 4,265,539 issued to Gaffard on May 5, 1981;
U.S. Pat. No. 4,352,565 issued to Rowe et al on Oct. 5, 1982; and
U.S. Pat. No. 4,495,411 issued to Rashleight on Jan. 22, 1985.
The Krogstad et al reference discloses an interference fringe movement detector for sensing interference pattern fringe movement or pattern shifts in which a radiation interference pattern is deflected by a galvanometer mirror to illuminate two photoelectric cells with selected portions of the interference pattern. However, none of the references suggest moving the front end of one fiber in an identical fiber pair to obtain both spatial and temporal coherence of an incident light field.
The Jacobs reference provides an optical comparator for measuring small surface vibrations on a rotating object using coherent optical techniques. By increasing coherence, the apparatus sensitivity is increased to the point where measurements on nonspecular surfaces can be achieved.
The Lebduska reference is an example of the use of side-by-side light conductors in a system for measuring packing graction and therefore the transmission efficiency of a fiber optic cable.
The Gaffard reference discloses a device which measures the mutual coherence function of a laser beam. A screen with openings is used to pass pencils of light taken from the laser beam. The pencils pass through a modulator and are then compared with a reference pencil of light. The average amplitude values and average phase values are representative of the amplitude and of the phase of mutual coherence function relating to the points of the cross section of the laser beam which correspond to the openings of the screen. This is described as a measure of the time coherence of the laser beam.
The Rowe et al patent discloses a speckle pattern interferometer employing a laser beam that is split into reference and object beams having substantially the same optial path lengths. The object beam is reflected from a vibrating object under investigation and the reference beam passes through an optical fiber cut to the proper length to equalize the length of the reference beam path with that of the object beam.
The Rashleigh patent demonstrates the use of identical side-by-side optical fibers to measure physical quantities. The fibers which are highly birefringent are oriented with their fast axes perpendicular to each other. When subjected to stress caused by a physical quantity acting on fibers through a transducer, the birefringence of each of the two fibers is asymmetrical altered thereby rotating the states of polarization of coherent light signals passing through the two fibers in a common direction. Environmental perturbations symmetrically alter birefringence of each of the fibers thereby rotating the states of polarization of the light signals in opposite directions. The states of polarization of the light signals emanating from the two fibers are detected and combined such that the changes due to the physical quantity enhance each other while the changes due to environmental perturbations are cancelled.
At present, the output of a large number of small laser beams are often combined so as to provide a single higher energy laser beam. In order to determine the output characteristics of the combined beam, a psuedo Youngs two aperture interferometer is employed to monitor the coherence of the radiation in the beams.
When utilizing known interferometer techniques, the beam to be monitored must be sampled by means of a mirror that is placed in the beam path to reflect a beam sample out to the interferometer used to measure coherence. This means that a disruptive fractor is placed in the beam path, i.e., a mirror having a cross section in the square inches range. In contrast to this, if the instrument of this invention is used, the cross sectional disruptments are only in the thousandnths of an inch. In order to measure the spatial and temporal coherence of an incident light field, the prior art normally requires the use of two distinct types of interferometers, e.g. Young's interferometer for spatial coherence, Michelson's interferometer for temporal coherence. The present invention solves this problem by measuring the spatial and temporal coherence of an incident light field with a single device.